Technical Field
The present disclosure relates to a phase-shifter circuit, or phase shifter, and to a corresponding method, for providing a ninety-degree (π/2) phase shift.
Description of the Related Art
There are several applications that rely on a ninety-degree phase shift of an input signal at a particular frequency value.
For instance, it is known that in MEMS (Micro-Electro-Mechanical Systems) gyroscopes a ninety-degree phase shift is used for carrying out open-loop cancelling of the so-called residual quadrature error. In this case, starting from a sinusoidal input signal at a given input frequency fz, generated within the gyroscope (the so-called “driver signal”), an output signal is to be obtained with a phase shift of 90° with respect to the input signal.
In a known way, the quadrature signal is proportional to the driving movement of the micromechanical sensing structure, differently from the angular-velocity sensing signal, which is, instead, proportional to the sensing movement (being a function of the Coriolis force). The quadrature signal may have an amplitude even considerably larger than the sensing signal and consequently is to be removed. The phase shift between the sensing signal and the quadrature signal is 90° (given that the two movements are mutually orthogonal). Consequently, the quadrature signal can be effectively removed using a phase-sensitive cancelling. Since even small phase errors can generate significant errors in cancelling the quadrature signal, it is important to obtain a high-precision ninety-degree phase shift.
For further details, reference may be made, for example, to the paper: “Open loop compensation of the quadrature error in MEMS vibrating gyroscopes”, R. Antonello, R. Oboe, L. Prandi, C. Caminada, and F. Biganzoli, IECON '09, 35th Annual Conference of Industrial Electronics, 2009.
It may moreover be important to maintain the ninety-degree phase shift also in the case where the value of the input frequency fz differs from the design value, for example due to a change in environmental conditions (with respect to temperature, pressure, mechanical stresses, or other factors) and/or due to variations in the manufacturing process.
Once again, this applies to the case of MEMS gyroscopes, where the input frequency fz of the aforesaid sinusoidal input signal may undergo variations, for example due to a change of the supply voltage value, the environmental conditions, or the manufacturing process.
As illustrated in FIG. 1, it may thus be important to maintain an exact 90° phase shift as the value of input frequency fz varies with respect to a nominal or typical value fz_typ, between a minimum value fz_min and a maximum value fz_max.
Known solutions for obtaining such a phase shift generally envisage use of a phase-locked loop (PLL), which is designed to lock to the input frequency fz of the sinusoidal input signal and generate an appropriate clock signal with a frequency fck that follows the aforesaid input frequency fz, as well as one or more switched-capacitor filtering stages with a cutoff frequency determined by the same clock signal.
By way of example, FIG. 2a illustrates a phase-shifter circuit of a known type, designated as a whole by 1.
The phase-shifter circuit 1 comprises a first lowpass-filtering stage 2 and a second lowpass-filtering stage 4, which are cascaded to one another and receive a sinusoidal signal with input frequency fz: A·sin(ωzt). In particular, both the first lowpass-filtering stage 2 and the second lowpass-filtering stage 4 are of the switched-capacitor (SC) type.
The phase-shifter circuit 1 further comprises a PLL stage 5, which receives the sinusoidal signal at its input and generates a clock signal synchronous with the same sinusoidal signal at a clock frequency fck locked to the input frequency fz, for example equal to N times the input frequency fz: fck=N·fz.
Given that the phase shift of each lowpass-filtering stage 2, 4 is equal to 45° (π/4) with respect to the corresponding cutoff frequency, it is sufficient for this cutoff frequency to be equal to the input frequency fz to obtain at the output a ninety-degree phase shift.
In a switched-capacitor filtering stage, the cutoff frequency can be set precisely in so far as it depends only on the clock frequency fck that regulates switching and on an internal capacitive ratio.
In particular, if the input frequency fz varies on account, for example, of the change of the environmental conditions, also the clock frequency fck, and consequently the cutoff frequency of the lowpass filtering stages 2, 4, varies accordingly, so that this solution is able to accurately follow the possible variations of the input frequency fz.
FIG. 2b shows a further known solution of a phase-shifter circuit, once again designated by 1, which differs from the solution described with reference to FIG. 2a in that it comprises a single bandpass-filtering stage 8, once again of the switched-capacitor type.
Also in this case, the PLL stage 5 generates the clock frequency fck locked to the input frequency fz in such a way that the central frequency of the bandpass-filtering stage 8 is equal to the input frequency fz, at which the ninety-degree phase shift is obtained.
Both described solutions, albeit enabling a ninety-degree phase shift to be obtained, in a way independent of possible variations of the input frequency fz, have the drawback of involving a considerable occupation of area and a considerable electric-power consumption, in particular on account of the presence of the PLL stage 5 and of two operational amplifiers (to obtain the lowpass filtering stages 2, 4 or the bandpass-filtering stage 8).
The above known solutions may hence not be usable in cases where a reduction in the energy consumption and the occupation of area are considerations (for example, in portable applications, such as in smartphones, tablets, or wearable electronic devices, e.g., smart-watches).